Now an archival no-no, laminating historical documents was the National Archives’ go-to preservation strategy for decades.

Lamination could be the cardinal sin of preservation. In a field where the golden rule is to do nothing you can’t undo, using heat to bond plastic to priceless documents is a major faux pas.

But in the early 20th century, lamination was a celebrated new tool in the conservator’s arsenal. Among its proponents: the National Archives and Records Administration (NARA) and the Library of Congress. From the late 1930s until the 1980s, NARA laminated what may total thousands of documents—including the Emancipation Proclamation and Louisiana Purchase paperwork. Now modern conservationists are trying to make the best of it.